Determination of refractive indices is frequently needed in industrial operations, typically in identifying a substance, measuring the concentration of a solution or liquid mixture, measuring a contaminant concentration in a certain material, and in monitoring the formation of a deposit or precipitate in a solution, the progress of reaction in a liquid, or the extent of polymerization reaction. For example, refractive index determination on site is in practice in the petroleum industry to determine the concentration of a petroleum component possibly mixed in an objective petroleum product, such as the concentration of butane that is likely to mix into octane during the production of the latter. The refractive index of octane is 1.39 and that of butane is slightly lower at 1.34. Mixing of butane into octane lowers the refractive index below the level of genuine octane, accordingly as the butane percentage increases. The refractive index reading thus tells how much butane has been mixed in. Process refractometers are in use elsewhere, e.g., in controlling the mixing of syrups with water for drinks and in controlling the polymerization processes for the preparation of polymers. Refractometers are also used in other fields, e.g., in the manufacture of pharmaceuticals, spices and flavors, oils and fats, fermentation products, and surfactants.
There are some known refractometers designed for the measurement of refractive indices. Abbe refractometer sandwiches a liquid whose refractive index is to be measured between the opposing inclined planes of two right-angled prisms, thus forming a liquid layer of about 0.1 mm thickness, and measures the angle of outgoing light through it corresponding to the critical angle. Being a transmission type, Abbe refractometer cannot be used for dark colored samples. It also requires the injection of a sample into between the opposing inclined planes of two right-angled prisms. For these reasons the refractometer is not practically useful for continuous monitoring at the site of commercial production.
An alternative approach is the refractometer of a total-reflection type. For example, a refractive densitometer for lubricating oils is marketed by Electromachine Co., the U.S., under the trade designation of "Model SSR-72." Light from a source is led through a condenser lens to form parallel light beams, while a motor-driven scanner with a spiraled slit revolves to scan the condenser lens, so that the light is injected into a bulk prism having a surface in contact with a fluid being measured, and the reflected light is determined by a detection unit. The instrument makes use of the phenomenon that light incident at angles smaller than the critical angle refract in a sample solution but the light rays at angles larger than the critical angle are totally reflected by a prism plane toward detection means.
A refractometer utilizing a bulk prism is on sale by ATAGO Co. as a process refractometer of PRM series. A block diagram of its detection unit is shown in FIG. 12 as a reprint from the manufacturer's Catalog Nos. 3621 and 3670. The detection unit is located on a part of a process line to detect the refractive index of the liquid flowing through the line. Light from a light source 30, such as a tungsten lamp or halogen lamp, is injected into a bulk prism 32. The bulk prism is trapezoidal, and the light is reflected on one side and is transmitted through or is totally reflected by a detection face 33 in contact with a sample liquid. In case of total reflection, the reflected beam is led through a lens and a light receiver 34 and is sent to an electric circuit 36. The electric circuit is connected with thermistors 40, 42, a humidity sensor 44, and a power supply circuit 46 so that it can yield refractive index, temperature, and various alarm outputs.
The above-mentioned refractive densitometer Model SSR-72 requires a lamp-type light source, motor-driven beam scanning mechanism, condenser lens, bulk prism, detector, etc. It therefore cannot be made small in size and is inconvenient for actual use. Another disadvantage is a large heat capacity. Further, because it includes moving parts, care must be taken in its operation and maintenance. ATAGO's process refractometer again includes a lamp-type light source and a bulk prism, which make size reduction impossible. When the bulk prism is used, the heat capacity is so large that much time is required before thermal stabilization is achieved, and hence a long measurement time is required. At a bulk prism, the light expands to such an extent that detection involves difficulties. The employment of a lamp as a light source causes a parallax, bringing the detection end (boundary) out of focus and reducing the measurement accuracy.
The present invention has for its object to develop a small, high-precision refractive index sensor of a total-reflection type which requires neither bulk prism nor lamp-type light source.